Without seeing a picture of exactly what's going on on the back of your coat, it's tough to diagnose the issue, as it could be any number of things. Darts should work, as long as the darts are at their widest (taking in the most material) where you are narrowest (at the waist).

Anyway, the major pattern manufacturers all have their catalogs available to browse online - Simplicity, McCall's, Butterick, and Vogue.